If you haven't seen my latest post, the dealer finally fixed the nav dvd issue. New nav unit took care of it. They opened a file with Toyota on it. Apparently only a few 2010 Prius are having this issue (even though the same nav unit is used in other Toy/Lex models). It was a pain but ended up with v10 software and a new Nav system, and no more annoying noise. If you turn off the radio, go into EV mode, turn off the heat/ac system, it's very obvious if you have the problem. My other thought was NavTraffic updates might be bombarding the system with data, but the new unit is quiet as a mouse, and this post will be the end of a 3 month new car project. Good luck, and hope you let Toyota know it's an issue on a few, as the "guru's" on Prius Chat don't know what I'm talking about.